From what I can tell, having done both Easy and Normal, the difference is the Ranks of the naturally spawning monsters.

Easy: Rank 1, 2 and 3 monsters spawn naturally.
Normal: Rank 2, 3 and 4 monsters spawn naturally.
Hard(Guess): Rank 3, 4 and 5 monsters spawn naturally.

Because of how the kill enough Rank X spawn a Rank X+2 monster system works spawning Stars/NM would be much easier in Hard.